Suspicious Package and Pacifist are great tools, but you can do it with the built-in pkgutil command:

    % pkgutil --expand foo-1.0.pkg foo_pkg
    % cd foo_pkg 
    % ls -Al
    total 352
    -rw-r--r--  1 magervalp staff  44491 16 Mar 12:19 Bom
    -rw-r--r--  1 magervalp staff    566  9 Apr 13:51 PackageInfo
    -rw-r--r--@ 1 magervalp staff  67794 16 Mar 12:19 Payload
    drwxr-xr-x  4 magervalp staff    128  9 Apr 13:51 Scripts
    % mv Payload Payload.cpio.gz
    % open Payload.cpio.gz